Depending on the intended application, there is an ongoing demand for smaller and lighter optical systems such as those used, for instance, in image display devices worn by an observer on his/her head (including the face), or in small cameras installed in mobile phones. There is also a progression towards higher pixel counts in imaging elements, as a result of high-image quality requirements, all of which is very demanding in terms of lens performance. The use of diffractive optical elements is one known means for meeting such requirements.
Basically, a diffraction grating is an optical element manufactured to have a grating structure with several hundreds of fine equidistant slits or grooves within a small gap (about 1 mm), such that when light strikes the grating, a diffraction beam is generated in a direction determined by the pitch (spacing) of the slits or grooves and by the wavelength of the light. The present invention aims at providing an optical element having an effect identical to that of such diffraction gratings, which are used in spectrometers and the like, and at applying the optical element in observation optical systems and/or imaging optical systems.
As these diffractive optical elements there have been proposed, in recent years, so-called multilayer diffractive optical elements. These diffractive optical elements, where plural diffraction element components having a saw-tooth shaped relief pattern are stacked on one another, allow ensuring high diffraction efficiency across most of a desired wide wavelength region (for instance, the visible region), i.e. they possess a good wavelength characteristic. Ordinary such multi-layer diffractive optical elements include, for instance, so-called bonded-multilayer diffractive optical elements that comprise two diffractive element components of mutually different materials, bonded to each other, with an identical relief pattern (as described in, for instance, Japanese Unexamined Patent Application Laid-open No. H09-127321).
Such a manufacturing method involves dripping an UV-curable resin, using a dispenser, onto a glass substrate where a relief pattern is formed. An UV-curable resin layer is then sandwiched between a third mold and the glass substrate, and thereafter UV radiation is irradiated to the UV-curable resin layer through the glass substrate, to cure the UV-curable resin. Demolding from the third mold yields then a diffractive optical element comprising the glass substrate and the UV-curable resin layer, with a relief pattern formed on the boundary surface thereof.
Despite the surface of the mold being flat, however, microscopic irregularities were apt to form on the surface of the UV-curable resin layer that is in contact with the surface of the mold. This was problematic in that formation of such irregularities on the surface of the UV-curable resin layer affected the optical characteristic of the diffractive optical element.